Kyrsten Sinema’s decision to run for Senate in Arizona is welcome news for Democrats

From left, Reps. Kyrsten Sinema (D-Ariz.), Katherine M. Clark (D-Mass.) and Joyce Beatty (D-Ohio) raise fists during a photo opportunity with the Democratic women of the House on Capitol Hill in Washington in January. Sinema is challenging Sen. Jeff Flake (R-Ariz.). (Cliff Owen/AP)

Next year's midterm elections don't give Senate Democrats a lot to look forward to. They're focused on trying to defend 10 senators up for reelection in states that President Trump won, in some cases by double digits.

The Democratic congresswoman from Arizona announced that she'll be challenging Republican Sen. Jeff Flake in the 2018 midterm election. This is a big deal for three reasons:

1. Flake is one of two vulnerable GOP senators up for reelection next year. The other is Dean Heller of Nevada, who also has a Democratic challenger.

2. Sinema is the Democrats' top choice to take on Flake. That's largely because she's good at raising money, and her moderate, even GOP-friendly politics matches the state's red-to-purple-ish profile. Having called an abandoned gas station home for three years as a child, she's also got a compelling life story, which is the focus of her campaign announcement.

Rep. Kyrsten Sinema (D-Ariz.) detailed her life story in her campaign announcement for Arizona's U.S. Senate seat. (Kyrsten Sinema)

3. Democrats needed a good candidate in Arizona to help them hold the line against Republicans and Trump. If they can pick up a seat in Arizona, it might make up for any number of elections in the Midwest where Democrats could lose. Republicans' 52-seat majority isn't really in question next November, but their ability to pass legislation by a bare majority is. Thanks in part to a divided Republican Senate, the united 48 members of Senate Democrats' caucus have managed to help stop two major pushes to repeal the Affordable Care Act.

Flake is by no means a goner. Arizona hasn't elected a Democrat to the U.S. Senate since 1988. In a state that isn't super fond of Trump, Flake has reputation as one of the president's most vocal GOP opponents, going so far as accusing his party of being in denial by supporting Trump.

But that's also earned him potential trouble in a primary, where Trump ally and former state senator Kelli Ward is challenging him. Trump himself has threatened to spend his own money to take out Flake.

Statements by Flake's campaign and Senate Republicans' campaign arm to the Arizona Republic indicate Republicans are going to try to paint Sinema as an unabashed liberal pretending to be a moderate.

“From her time working on Ralph Nader's campaign to the state legislature to Congress, Kyrsten Sinema has always been out of touch with Arizona, and she'll do anything to hide her progressive record,” Flake campaign spokesman Will Allison said.

But her supporters point out that Sinema didn't vote for Nancy Pelosi as Democrats' House leader, and she skipped out on a Hillary Clinton campaign rally.

“We can change a broken Washington and make it work again,” she said in her campaign announcement.

There are data points both sides throw out to show why they can win this race. Democrats say Arizona is trending to them very quickly. Trump only won it by three points. But Sen. John McCain (R-Ariz.) easily won reelection last year against Ann Kirkpatrick, a Sinema-esque candidate.

An easier way of translating all this is: Arizona now looks like it's going to be one of the nation's most competitive Senate races.

And in a year where most of the competitive races are against Democrats, they'll gladly take the chance to go on the offensive with a top-tier candidate.